This invention relates to mass data storage devices. In particular, this invention relates to sets of physical mass storage devices that collectively perform as one or more logical mass storage devices. More particularly, this invention relates to improving the flexibility, transaction/bandwidth performance, speed and reliability of such sets of physical mass data storage devices.
It is known that a plurality of mass data storage devices can be operated as a single logical mass data storage device. Such devices can be disk drives or any other mass data storage technology, including tape or other technology. When a set of such devices is operated collectively as a logical device, data stored in a write operation can be spread across one or more members of the set.
One of the advantages of such sets is that data can be stored or retrieved faster because they can be written or read in parallel to or from all the physical devices. Another advantage of such sets is that they may be more tolerant of physical failures. For example, if one physical device fails, it may still be possible to read and write the other members of the set. Further, by storing redundant information in the set, one can increase the probability of being able to recover data stored on a failed physical device. The redundant information may take the form of mirrored or shadowed data, in which case data stored on any device of the set is duplicated on another device of the set. Alternatively, the amount of redundant information stored in the set may be reduced by storing what is commonly referred to as check data. Such check data typically comprises code words that are constructed by encoding the data stored in the set. Reconstruction of data that was stored on a failed device is attained by mathematically extracting the missing data from the code words.
Another advantage of using a plurality of physical devices as a logical device is that the ratio of read/write heads per unit of mass storage is increased. As a result of having a greater density of such actuators each capable of independently accessing data, it becomes possible for the logical device to handle a larger number of data requests simultaneously. This is advantageous in applications such as data bases and on-line transaction processing that require high aggregate request/second rates.
The use of a number of physical devices in parallel can, depending on the configuration, also give such sets potentially high bandwidth--i.e., a lot of data can be written or read at the same time. High bandwidth is advantageous for some applications, such as real-time analysis, numerical analysis and image processing. A known technique for providing high bandwidth in such a device set is to store data by interleaving data blocks across the devices of the set, such that the blocks are sequentially stored in round robin fashion throughout the set. This is also known as striping the data.
However, while sets of physical mass storage devices potentially have the above advantages, among others, known techniques for organizing and backing up data in such sets do not provide flexibility in optimizing these advantages for different applications. They also do not exploit the full range of data organizations that are possible in such a device set. In other words, a mass storage apparatus made up of a plurality of physical storage devices may be called upon to operate as a logical storage device for two concurrently-running applications having different data storage needs--for example, one application requiring large data transfers (i.e., high bandwidth), and the other requiring high frequency transfers (i.e., high operation rate). A third application may call upon the apparatus to provide both high bandwidth and high operating rate. Known operating techniques for physical device sets do not provide the capability of dynamically configuring a single set of physical storage devices to provide optimal service in response to such varied needs.
It would therefore be desirable to be able to provide a mass storage apparatus, made up of a plurality of physical storage devices, which could flexibly provide both high bandwidth and high operation rate, as necessary, along with high reliability.